1. Field of the Invention
The present invention relates to an immersion lithography apparatus and an exposure method.
2. Related Background Art
In an immersion lithography apparatus, a resist film or the like is formed on a processing target substrate, and then exposure is carried out while immersion fluid is flowed between the substrate and a lens (an optical element) of the lithography apparatus. As the immersion fluid, for example, pure water or pure water having a gas or a chemical added thereto as required is used. Since impurities in this pure water have various influences on an exposure step and the lithography apparatus, impurities concentration must be set to a fixed value or below. For example, it is known that an increase of an organic material concentration in the pure water would cause a temperature fluctuation during exposure to degrade image forming performance or deposition of the organic material onto a surface of the lens (the optical element) to provoke long-term illumination intensity unevenness/degradation or reduction of illumination intensity.
In the conventional immersion lithography apparatus, in order to manage impurities concentration in pure water that is introduced into the lithography apparatus, a pure water manufacturing unit is installed immediately before the lithography apparatus to monitor, e.g., a TOC (Total Organic Carbon) concentration, a dissolved gas concentration, or a specific resistance value in the pure water at an introduction opening leading to the lithography apparatus when a purity of the pure water to be supplied is insufficient.
Even though a purity of the pure water to be supplied is adequate, however, there occurs a problem that performance of a wetted lens of the lithography apparatus degrades or exposure performance degrades when impurities are eluted from the processing target substrate.
Although there is a conventional technology of performing pure water rinse to remove impurities before applying a resist to a wafer, or after applying the resist, or after forming a protective film, this pure water rinse is mainly implemented by using ordinary pure water in a resist application unit. As immersion fluid utilized in immersion exposure, pure water with a very high purity obtained by further removing impurities from pure water or pure water having a fixed gas or chemical added thereto is used. Therefore, solubility of impurities from the processing target substrate may be different from that of the ordinary pure water in some cases. Further, when the immersion fluid is used during immersion exposure processing, a pressure is applied to the resist or the protective film depending on conditions, e.g., a water pressure or a flow rate, and elution of impurities may be facilitated. Therefore, when immersion using ordinary pure water or cleaning using immersion fluid alone is performed, impurity elution during exposure may not be sufficiently reduced.
Furthermore, there is also a proposal of monitoring impurities in recovered water when immersion fluid is flowed on an exposure stage (e.g., WO 2006/062188). This is effective for monitoring impurities, but the impurities are already in contact with the lens when the impurities are detected during monitoring, and there is a problem that contamination of the lens advances.